This invention relates to sliding seal and cylinder liner compositions for adiabatic engines. Specifically, this invention relates to a stable ceramic composition having a surface with a low coefficient of friction and a high wear resistance at high operating temperatures and a method for producing this composition.
This invention arose under United States Government Contract No. DEN3-352 and the United States Government has a nonexclusive, nontransferable, irrevocable paid-up license to practice or have practiced for or on behalf of the United States this invention throughout the world.
There has been great interest in developing an uncooled, ceramic diesel engine since ten to thirty percent of the work output of an internal combustion engine is expended on the cooling system. Nearly adiabatic operation and recovery of exhaust heat in a turbocompounding system may increase the thermal efficiency of such an engine to 48%. Additional benefits may include multi-fuel capability, reduced noise, reduced white smoke emissions, lower compression ratio, and reduced maintenance. Further, such an engine may be lighter and easier to produce because of the elimination of the need for a cooling jacket. In comparison to the overall effort on ceramic diesel engine development, little work has been done in the important area of developing a sliding seal/cylinder liner system capable of performing adequately under harsh conditions.
This invention relates to providing promising ceramic materials for piston rings and cylinder walls in the abiabatic diesel engine. There has been some progress in development of ceramic pistons (or piston caps) and cylinder liners for adiabatic engines but most designs involve a piston cap in a crosshead engine using conventional chromium-plated cast iron rings and low-temperature lubrication, which offers a very low coefficient of friction (about 0.05) and low wear rates. However, in a radiatorless engine, the temperature of the metal is likely to become unacceptably hot. Also, this type of design is bulky, involving extra piston weight and high inertial forces to the connecting rod as well as on the joint between the piston and the ceramic piston cap.
On the other hand, a simpler design in which the entire cylinder is cermic-lined requires that the piston rings slide against the ceramic liner, which will be quite hot. There is an urgent need to find materials from which sliding seal rings can be manufactured which will exhibit low friction and low ring/liner wear while sliding against a ceramic cylinder liner at high velocity and high temperature.
The piston crown temperature must be raised to over 880.degree. C. in order to achieve significant energy savings in the adiabatic diesel. Thus the prime requirement of the cylinder liner/seal ring system is resistance to thermal shock and retention of strength to temperatures much higher than that which the top ring in a conventional diesel may experience. The liner/ring combination must exhibit acceptable wear rates not only at high temperatures, but also at high velocities of as much as 5-10 meters per second, and under conditions of exposure to combustion products and excess oxygen. Low friction is essential. The liner materials of interest have been silicon carbide, silicon nitride, and partially-stabilized zirconia. The silicon carbide, although it has been used for solid lifters and precombustion chambers in the heavy-duty diesel program, has a high thermal conductivity, which reduces its usefulness for piston crown and cylinder liner applications.
Silicon nitride has been the material of choice in most recent work, because of its low thermal conductivity, relatively high strength at temperatures to about 1100.degree. C., resistance to thermal shock, and relative ease of manufacturing complex shapes. However, in the push for higher temperatures still, the limits of silicon nitride are already being approached. Interest exists in using zirconia because it is strong even at 1500.degree. C. and has better insulating properties than silicon nitride.
Although there is little experimental data on friction and wear of ceramics under conditions that would be experienced by the sliding piston seal in an advanced diesel engine, the information that is available is fairly consistent in that the carbides are identified as outstanding sliding seal materials. In work done by Finkin, Calabrese, and Peterson, as noted in ASLE Preprint #72LC-7C-2 (1972), oscillatory pin-on-plate sliding experiments were performed with various ceramic/cermet couples in air at 316.degree.-982.degree. C. A nickel molybdenum (Ni-Mo) bonded titanium (TiC) carbide cermet, sliding on dense magnesia-stabilized zirconia, was identified as the most promising material couple. A poor second was the nickel-molybdenum (Ni-Mo) bonded titanium (TiC) sliding on itself. Zirconia sliding on itself suffered catastrophic wear.